In the field of orthopedics, the cutting of living bone is a prerequisite for many procedures. Such procedures include the reconstruction of damaged tissue structures due to accidents, the grafting of healthy bone into areas damaged by disease, or the correction of congenital facial abnormalities like a receding chin line. Over several centuries, these tasks were performed through the utilization of devices called bone saws.
Traditional bone saws are categorized into several basic categories. Hand powered saws or drills are just that, hand held devices which require the operator to move the device in a fashion similar to that used for carpentry tools. Powered devices, whether electric or pneumatic, are of either the reciprocating or rotary type. The reciprocating devices use a flat, sword like blade where the back and forth motion is provided by a motor instead of the hand. The rotary devices use a rotating motor to spin a drill bit or a blade which has teeth arranged around its circumference similar to a table saw blade. All of these traditional bone saws are used today in medical procedures around the world.
In many surgical operations it is necessary to obtain direct access to the cranial cavity and the brain. To perform such operations it is often necessary to drill holes through the skull bone. Since the bone is very hard, it is necessary to apply significant pressure to drill through it. Since the dura beneath the skull bone and the brain itself are very delicate, it is difficult to prevent the dura from being cut or damaged when using conventional rotary drills, whether manually or automatically powered and controlled.
In the past, surgeons have used hand braces and bits of a design very similar to those used for non-medical purposes, for example carpentry. Such tools are not completely satisfactory because it has been found that such tools can cut through the skull and damage the meninges or brain and tend not to leave the skull or the underlying membranes in a condition that enables them to heal to approximately their original condition.
It has been found that ultrasonic blades, if properly designed and properly used, can cut bone without damaging the soft tissue adjacent the bone. U.S. Patent Application Publication No. 20050273127 by Novak et al. discloses a surgical blade and a related method of use of that surgical blade in an ultrasonically assisted procedure for cutting bone, wherein adjacent soft tissue is not damaged. The observation was made that the sharper the blade, i.e., the smaller the minor dimension of a vertical trapezoid formed by the included angle of a blade of width N, the more likely that cutting of hard tissues resulted in collateral damage, particularly incisions, in surrounding soft tissue. It was discovered that blades with an edge thickness between approximately 0.001″ and approximately 0.010″ inch offered the best compromise between effective, safe cutting of hard tissue such as bone while being sparing of surrounding soft tissue.
The teachings of U.S. Patent Application Publication No. 20050273127 pertain to linear cutting blades moved by a reciprocating sawing-type motion, and not to rotary tools. Drilling into bone evidently requires its own protective technique and associated tool for minimizing or avoiding damage to brain tissues.